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When Bobby, of Warren, gets hurt, his body reacts much like anyone's - cells move in to repair and replace other damaged cells. The difference is his body replaces damaged muscle, ligament and tendon tissue with bone.

FOP causes bone to form when muscles, tendons and ligaments are damaged.

Over time, bone replaces the other tissues. The results are increasing loss of mobility, pain, and, in the worst case, the formation of a second skeleton - effectively trapping a person in his or her own body.

There are fewer than 800 known cases worldwide.

With so few cases, and only a few decades of historical medical records, much is not known about the disease.

Bobby's father, Kyle, said scientists recently identified the portion of the gene that is responsible for the disease. "They've got the science of the thing figured out," he said. "Now it's what to do to control it."

There is no known treatment nor cure. Corticosteroids can be used to reduce the risk that an injury will result in bone growth and the Johnsons keep some handy.

Because it is so rare, Bobby's condition was not accurately diagnosed at first.

The malformed big toes that are one of the tell-tales of FOP were surgically repaired. The first flare-up - a large mass that appeared suddenly on his back shortly after his first birthday - was also not recognized as FOP.

"This mass had taken over his back," Bobby's mother, Carmen, said.

"His whole back was a flare-up," Kyle Johnson said. "From his shoulder down to his tailbone."

The mass stuck out almost three inches.

Doctors took a biopsy - a cut they did not know would be repaired with bone.

"It came back that he had a sarcoma," she said.

"That was devastating," Kyle Johnson said.

The doctors said Bobby had about one chance in 20 of surviving five years.

That the diagnosis was wrong was good news. The correct diagnosis, suggested by a pathologist who remembered that Bobby had had toe surgery, was not.

"In the first year we were helicopter parents," Carmen Johnson said.

"Every day was stressful," Kyle Johnson said. "As a toddler, you're going to fall down. With that there would be the flare-ups. We just lived with that fear."

He said there is an overwhelming chance - 90 percent - of a flare-up when a person with FOP is injured.

A flare-up does not automatically cause further problems.

"It won't necessarily leave bone behind," he said.

Nor are they always painful.

Some patients report that their flare-ups are "extremely painful," Kyle Johnson said. "(Bobby) doesn't ever say that any of them hurt."

Injury is not the only case under which Bobby may grow more bone.

"It can be from an injury," Carmen Johnson said. "It can be spontaneous."

One way or the other, bone will build up in his body.

"Down the road it's just going to get worse," Carmen Johnson said. "Complete immobilization would be the worst-case scenario."

Some people with FOP end up completely immobilized; others have limited motion, she said. "It varies from patient to patient. That's why it's a preventative maintenance thing."

Bobby is already suffering some of the effects of his FOP. He has limited range of motion in both his shoulders and neck.

Doctors have distributed questionnaires in an effort to develop some baseline information on FOP. They are also asking patients to undergo daily urinalysis to see if some change in nutrient content precedes flare-ups.

Clinical trials could begin soon, Kyle Johnson said. Patents have been granted and successful experiments on a colony of mice have been completed.

While he knows the experts are doing their best, he would like to see the steps coming faster.

"We don't really see it, but they're working very hastily on a number of different things as we speak," Kyle Johnson said. "They need that to start the clinical trials."

Bobby's parents are still protective, but, with the help of an online community of families of FOP patients, they've learned not to hover so close.

"They loved that their parents let them be kids," she said. "That's been a huge help to us."

Now, "if it's something simple, we just stand near him and let him be a child," she said. "He does want to go out and play soccer. We let him play in the back yard."

Bobby attends pre-school at Don Mills Achievement Center and his parents expect to enroll him in kindergarten in the fall.

He can play with his siblings - 9-year-old Kylie and 10-month-old Addison.

"We do let them play," Kyle Johnson said. "We try not to limit him too much."

"Sometimes we'll get into a little wrestling match, but it's controlled," he said. "We make the most of it."

There are limits.

"I'm not going to let him jump on his bed... there's no way," Kyle Johnson said. "I'm not going to let them throw things at each other."

Both parents stated their objections to trampolines.

The limits are not so strict that Bobby feels different.

"If you ask him, he feels like a normal kid," Carmen Johnson said. "He knows what he can and can't do."

"Luckily, he's more into technology," she said. "He's not as wild as your typical four-year-old. He's found other things that he likes."

In addition to playing video games and driving his Power Wheels, Bobby loves baking, his mother said.

He is allowed to try new things, even when there is some risk of bumps and bruises. His parents have established that that risk is less than the possibility of denying their son a childhood.

"It's been more beneficial to him in the long-run if we give him the option," Kyle Johnson said. "I would rather him feel that he can do anything in the entire world."